Spherulitic growth and cellulation in dilute blends of monodisperse long n-alkanes

Dilute blends of two guest monodisperse n-alkanes one, n-C122H246, shorter and one, n-C246H494, longer than the pure host, n-C162H326, have been investigated regarding the kinetics and morphology of both spherulitic growth and cellulation to clarify the related phenomena in polymers. All samples were crystallized isothermally between 118 and 124 degreesC. For the pure material this gave the optical textures expected of chain-extended crystallization, i.e., objects which may display circular envelopes but differ from classic spherulites in that they have domains of uniform crystallographic orientation. With increasing crystallization temperature these objects become larger and coarser to a limit when they are no more than aggregates of single crystals. Addition of 5% or 10% of a second component to n-C162H326 lowers growth rates and drastically alters morphologies. When the guest is longer than the host, there is cocrystallization producing permanent cilia and finer-textured, radially symmetric spherulites. When it is shorter, it segregates with a decelerating growth rate akin to observations of cellulation in branched polyethylenes. The resulting solids have disrupted crystallographic ordering and textures with randomly oriented lamellae. In all cases, adjacent dominant lamellae splay apart by angles which increase linearly with supercooling. The rate of increase is less for blends than for the pure host and the magnitudes increase with guest concentration being greatest when there is both permanent and transient ciliation. Positive intercepts at zero supercooling may indicate a supplementary contribution from surface roughness of the embryonic lamella. The combined observations provide strong confirmation of the importance of ciliation in promoting spherulitic growth of long molecules, independent of segregation and cellulation.